\documentclass[../libro.tex]{subfiles}

\begin{document}

\ifSubfilesClassLoaded{\backmatter}{}

\clearpage

\ifodd\value{page}\else%
\mbox{}\thispagestyle{empty}\clearpage%
\fi

\mbox{}\thispagestyle{empty}\clearpage

\thispagestyle{empty}
\ifdeflength{\centeroffset}{}{\newlength{\centeroffset}}
\setlength{\centeroffset}{-0.5\evensidemargin}
\addtolength{\centeroffset}{0.5\oddsidemargin}
\vspace*{\fill}
\noindent\hspace*{\centeroffset}\makebox[0pt][l]%
{%
    \begin{minipage}{\textwidth}
        \centering\itshape\Large%
        \esperanto{Enkonduko al determinantoj}%
    \end{minipage}%
}\par%
\vspace{.5\baselineskip}\mbox{}\vfill

\end{document}

I do not know (or rather, I forgot)
what inspired this back cover....
